In catalysts, cobalt is normally fixed on a carrier such as silica, aluminium silicate, alumina.
In these catalysts, the useful cobalt atoms are those which are exposed at the surface of the cobalt particles. The cobalt atoms which are not exposed (i.e not at the surface) will not participate in catalytic reaction.
Cobalt is an expensive metal and, in order to optimize its use as a catalyst, it is essential to increase as much as possible the (number of exposed cobalt atoms/ total number of cobalt atoms) ratio of the catalyst which, in turn, increases the cobalt surface area per gram of cobalt.
It is known from EP-A-13,275 to produce a supported coprecipitated cobalt-silica catalyst wherein a reaction mixture of cobalt ions, silicate ions and solid porous carrier particles is prepared and allowed to form a coprecipitate of cobalt and silicate ions onto the solid porous support particles. The obtained cobalt-silica catalyst has a BET total surface area ranging from 150 to 350 m.sup.2 /g and a cobalt surface area ranging from 5 to 20 m.sup.2 /g of cobalt.
It is also known from U.S. Pat. No. 4,591,579 to provide a process for the preparation of a transition metal-silicate catalyst in which an insoluble, basic compound of a transition metal (e.g. cobalt, nickel or copper) is precipitated with an alkaline precipitation agent from an aqueous solution of such a metal salt after which this compound is reacted with a silicate solution. In example 5, it is described such a catalyst wherein the cobalt surface area is 8.9 m.sup.2 /g of catalyst.
In `Stoichiometries of H.sub.2 and CO Adsorptions on cobalt`--Journal of Catalysis 85, page 63-77 (1984)--are disclosed on page 67, table 1, cobalt catalysts on different carriers. From the total maximum H.sub.2 uptake, it is possible to calculate the cobalt surface area per gram of catalyst and the cobalt surface area per gram of cobalt. It can be seen that, for cobalt on silica catalysts the cobalt surface area per gram of cobalt ranges between 6 and 65 m.sup.2 /g whereas for cobalt on transition alumina catalysts the cobalt surface area per gram of cobalt ranges between 15 and 26 m.sup.2 /g.
Thus, cobalt catalysts with a high cobalt surface area per gram of cobalt exist for cobalt on silica catalysts (and also for cobalt on carbon catalysts), but don't exist for cobalt on transition alumina catalysts.
Nevertheless, cobalt upon transition alumina catalysts present some distinct advantages towards other cobalt catalysts.
First of all, a cobalt on transition alumina catalyst is easier to shape by extrusion than a cobalt on silica catalyst and the mechanical strength of the resulting catalyst is higher.
In reactions where water is present (e.g. methanation, Fisher Tropsch), silica can be unstable. Alumina, however, is much more stable under such conditions.
There is therefore a need for a cobalt on transition alumina catalyst with a cobalt surface area per gram of cobalt higher than previously obtained.
Its is a first goal of the present invention to provide a cobalt on transition alumina catalyst with a cobalt surface area per gram of cobalt higher than previously.
It is a second goal of the present invention to provide a process for manufacturing such catalyst.
Tests and Definitions
i) cobalt surface area
Approximately 0.5 g of sample is used for the analysis. The weight used to calculate the metallic surface area is that obtained after pretreatment. During this pretreatment the sample is degassed and dried under vacuum at 120.degree. C. The pretreated sample is then reduced. Sample is heated to 425.degree. C. at a rate of 3.degree. C./min whilst hydrogen gas is passed through the sample at a flow rate of 250 ml/min. Still with the same hydrogen flow the sample is maintained at 425.degree. C. for 18 hours. Under vacuum the sample is heated up to 450.degree. C. over a 10 min time period. The sample is maintained at 450.degree. C. under vacuum for 2 hours. PA1 The chemisorption analysis is carried out at 150.degree. C. using pure hydrogen gas. An automatic analysis program is used to measure a full isotherm up to 800 mmHg pressure of hydrogen. PA1 The method is to extrapolate the straight-line portion of the chemisorption isotherm between 300 and 800 mmHg to zero pressure to calculate the volume of gas chemisorbed (V). PA1 Metallic surface areas were calculated in all cases using the following equation, ##EQU1## Where V=uptake of hydrogen in ml/g SF=Stoichiometry factor (assumed to be 2 for H.sub.2 chemisorption on Co) A=area occupied by one atom of cobalt (assumed to be 0.0662 nm.sup.2) PA1 This equation is disclosed by Micromeretics in Operators Manual for ASAP 2000 Chemi System V 1.00, Appendix C, Part No. 200-42808-01, 18 January 1991. PA1 Transition aluminas are defined in "Ullmans Encyklopaedie der technischen Chemie", 4., neubearbeitete und erweiterte Auflage, Band 7 (1974), pp.298-299. PA1 The document divides transition aluminas in several categories: PA1 -gamma-group PA1 delta-group
ii) Transition alumina
Included in the gamma-group are, apart from gamma-Al203, all low-temperature forms such as eta-A1203 and chi-A1203. They are formed on calcination of aluminiumhydroxides at 400.degree.-750.degree. C. PA2 The specific surface area of gamma-group forms of aluminas is in the range of 150-400 m.sup.2 /g. PA2 The delta group of aluminas includes all high-temperature forms, e.g. delta-, theta- and chi-Al203. The delta group aluminas are formed on heating gamma-group aluminas at approximately 800.degree. C. or higher. PA2 The specific surface area of delta-group forms of aluminas is in the range of 50-150 m.sup.2 /g.
General Description of the Invention
It is a first object of the present invention to provide a cobalt, on transition alumina support, catalyst, containing between 3 and 40% by weight-of cobalt wherein the cobalt surface area is above 30 m.sup.2 /g of cobalt, preferably above 40 m.sup.2 /g, more preferably above 50 m.sup.2 /g of cobalt, even more preferably above 80 m.sup.2 /g of cobalt.
Preferably, the transition alumina support is a gamma alumina or a theta alumina, more preferably a theta alumina.
Preferably, the cobalt catalyst contains 5% to 20% by weight of cobalt, more preferably 10 to 20 % by weight.
It is a second object of the present invention to provide a process for manufacturing a cobalt, on transition alumina support, catalyst, containing between 3 and 40% by weight of cobalt, the cobalt surface area being above 30 m.sup.2 /g of cobalt, wherein a slurry of transition alumina in an aqueous solution of cobalt ammine carbonate is heated to a temperature of 60.degree. C. to 110.degree. C., in order to allow cobalt hydroxycarbonate to precipitate, the resulting product being then dried and calcined. Optionally, the calcined product may be further reduced.
Preferably, the transition alumina support is a gamma alumina or a theta alumina, more preferably a theta alumina.
It is a third object of the present invention to provide a process for manufacturing a cobalt, on transition alumina support, catalyst, containing between 3 and 40% by weight of cobalt, the cobalt surface area being above 30 m.sup.2 /g of cobalt, wherein, transition alumina particles are saturated with an aqueous solution of cobalt ammine carbonate, the excess solution being removed by filtration, the resulting product being heated to a temperature of 60.degree. C. to 110.degree. C. , in order to allow cobalt hydroxycarbonate to precipitate, the resulting product being then dried and calcined.
Preferably, the transition alumina support is a gamma alumina or a theta alumina, more preferably a theta alumina.
Successive impregnation and precipitation steps may be applied to increase the cobalt content, the deposited cobalt hydroxycarbonate being converted into cobalt oxides during a calcination treatment at a temperature of 200.degree. to 600.degree. C.
The product can then be activated with hydrogen gas at temperatures between 200.degree. and 600.degree. C. preferably between 350.degree. and 550.degree. C. and then optionally passivated
Specific Description of the Invention